1. Field of the Invention
The present invention relates to pyroelectric infrared detectors having decreased noise levels without reduction in the responsiveness of the pyroelectric infrared detector.
2. Description of the Prior Art
Pyroelectric infrared detectors (referred to as "pyroelectric detector" herein) are roughly sorted into two types depending on the method for picking up output from pyroelectric element: namely, a first voltage mode type and a second current type. In most uses, however, the pyroelectric detector is used in the voltage mode. The present invention is concerned with an improvement in the voltage circuit mode. There are three basic types of internal circuit of the pyroelectric detector, as shown in FIGS. 4(a) to 4(c). More specifically, FIG. 4(a) shows a single type pyroelectric element in which one of the electrodes of a single pyroelectric element is connected to the gate of a FET 3. This is the basic form of the voltage mode pyroelectric detector. A reference numeral 2 denotes a resistor having a high electric resistance on the order of 1.times.10.sup.9 to 1.times.10.sup.12 .OMEGA.. The resistor 2 serves as a leak resistor which prevents saturation of the gate of the FET 3 in the event of an excessive input to the detector. The resistor 2, however, may be omitted because the pyroelectric element 1 itself serves as a leak resistor when the resistance value of the pyroelectric element ranges between 10.sup.9 and 10.sup.12 .OMEGA.. FIG. 4(b) shows a pyroelectric detector generally referred to as dual type or twin type. This type of detector employs a composite pyroelectric element 4 composed of a pair of pyroelectric elements 1,1' having opposite polar directions and connected in series to each other. In this pyroelectric detector, only one of the pyroelectric elements 4 receives infrared rays. This arrangement effectively negates any erroneous signal which may otherwise be caused by vibration or fluctuation in the ambient temperature so that the error signal is not transmitted externally. Thus, this type of pyroelectric detector is suitable for use in the cases where a specifically high reliability is required, e.g., in an intrusion alarm.
Still another type of pyroelectric detector is shown within the broken lines in FIG. 4(c). In this pyroelectric detector, the source terminal of the FET 3 is connected to the voltage reference point so that signal output is derived from the drain terminal. This circuit enables the bias point of the output terminal to be set freely by suitable selection of the drain resistance 5, even when the leak resistor 2 is not provided. It is, therefore, possible to avoid any malfunction of the detector attributable to the saturation of the FET 3 in the event of an excessively large input to the detector. Since the pyroelectric element 1 has a high impedance on the order of 10.sup.11 to 10.sup.13 .OMEGA., the FET 3 provides impedance conversion for converting the generated signal into a signal of low impedance.
The performance of the pyroelectric detector can be evaluated in terms of voltage responsivity and noise level. The factors of the voltage sensitivity and the noise will be described hereinunder.
According to E. H. Putley, Semiconductors and Semimetals (edited by Willardson) 5, p 259, Academic Press (1970)), the level of the voltage responsivity is expressed by the following formula: EQU R.sub.v =.eta..multidot..omega..multidot.A.multidot.R/G.multidot.dP.sub.s /dT.multidot.1/.sqroot.1+.omega..sup.2 .tau..sub.T.sup.2 .multidot.1/.sqroot.1+.omega..sup.2 C.sup.2 R.sup.2 ( 1)
where, .eta. represents the radiation factor, .omega. represents the chopping angular frequency (=2.pi.f, f being chopping frequency), A represents area of light-receiving electrode of element, G represents radiative conductance, dP.sub.s /dT represents pyroelectric coefficient, .tau..sub.T represents thermal time constant (=H/G, H being heat capacity), C represents equivalent input capacitance of the detector, and R represents the equivalent input resistance.
The equivalent input capacitance C and the equivalent input resistance R are considered as being the synthetic values of the capacitances and resistance values of the pyroelectric elements 1,4, leak resistor 2 and the FET 3. Practically, however, the equivalent input capacitance C is substantially the same as the capacitance of the element, while the equivalent input resistance R is substantially the same as the leak resistor.
The noise of the pyroelectric detector also has a plurality of factors. These factors are, for example, temperature noise caused by the fluctuation in the ambient temperature, tan .delta. noise attributable to the dielectric loss of the pyroelectric element, input resistance noise (referred to also as Johnson noise) caused by the input resistor R, FET noise caused by gate leak current of the FET and FET voltage noise. The level of the noise of the pyroelectric detector is determined as the square mean of these factors. The inventors have made an intense study to analyze the effects of the respective factors and found that the input resistance noise is most dominant and the FET current noise comes next. It has thus proved that the noise of the pyroelectric resistor can satisfactorily be discussed only on the basis of these two types of noise factor. The level of the noise, therefore, can practically be expressed by the following formula. ##EQU1##
where, k represents Boltzmann's constant, T represents the absolute temperature and i.sub.n represents the FET leak current.
Obviously, it is preferred that a pyroelectric detector has a responsivity which is as high as possible and a noise level which is as low as possible. Practically, however, it is quite difficult to attain a design which satisfies both these demands. Therefore, it has been a conventional measure that either one of these demands is satisfied preferentially while the other is obliged to be compromised. More specifically, effort has been concentrated to the enhancement of the responsivity, rather than to the reduction of the noise level. Namely, noise has been accepted as being unavoidable, provided that it can suitably be processed. In recent years, however, there is an increasing demand for reduction of the noise to a level below a predetermined limit, in order to cope with the current trend for sophistication of various appliances in this field.
From the formula (2) mentioned before, it is understood that the noise can be reduced by adopting large values for the equivalent input capacitance C and the equivalent input resistance R, while using a FET having a small level of leak current i.sub.n. Thus, if the FET is given, only the factors C and R are selectable. The factors C and R are the equivalent capacitance and the equivalent resistance as viewed from the output side of the pyroelectric detector which is composed of the pyroelectric elements 1,4, leak current 2 and FET 3. Practically, however, the factor C is determined by the capacitance of the pyroelectric elements 1,4 which in turn is determined by the dielectric constant .epsilon..sub.r, thickness of the element and the area of the electrode. Similarly, the factor R is materially determined by the resistance value of the leak resistor 2. From the viewpoint of reduction in the noise level, the equivalent input resistance R is preferably made as high as possible. The resistance value of the leak resistor R, however, cannot be increased unlimitedly because the leak resistor 2 has to leak electrode charge in the event of an excessively large input, in order to prevent saturation of the FET. Thus, in the practical pyroelectric detector, the upper limit of the resistance value of the leak detector R is selected to be about 5.times.10.sup.11 .OMEGA.. In regard to the capacitance C, it is to be noted that no proposal has been made up to now for controlling the capacitance C for the purpose of reducing the noise level. The reason why such a proposal has not been made is that the control of the capacitance C leads to a reduction in the responsivity. As will be seen from formula (1), the voltage responsivity R.sub.v is decreased substantially in inverse proportion to the capacitance C. In other words, it is essential that the capacitance component of the pyroelectric element be reduced for the purpose of increasing the responsivity of the pyroelectric detector. In the conventional design of pyroelectric detectors, therefore, effort has been focused on the reduction of the capacitance component other than the capacitance component of the pyroelectric element itself. Thus, no approach has been made to the reduction in the noise through a control of the capacitance component, and attempts for reducing the noise in pyroelectric detectors have relied only upon the control of the resistance value of the leak resistor 2. In consequence, there has been a practical limit in the reduction in the noise level.
Another method for reducing the noise level is to select and use a FET having small leak current value i.sub.n. This effectively reduces the term of the current noise in formula (2). Such a FET, however, is generally expensive and, in addition, has an inferior signal transmission efficiency which is as low as about 50 to 60% of the FET of ordinary specification. Thus, the use of such a FET seriously reduces the sensitivity when used in a pyroelectric detector, i.e., reduces the S/N ratio (signal to noise ratio) undesirably. In addition, it has been almost impossible to control the noise to a desired level.
Under these circumstances, the present inventors have made an intense study in order to develop a pyroelectric detector which is improved to reduce the noise level without being accompanied by any deterioration in the responsivity, by throughly reviewing the conventional concept in this field of technology.